a. Field of the Invention
The present invention relates to an echo canceller for cancelling an echo signal (echo) produced in a telephone circuit or a hands-free telephone set and a communication apparatus using it.
b. Background Art
A telephone set which is capable of freeing hands of a user and allows talking by using a microphone and listening through a loudspeaker, instead of using a conventional handset has been recently popularized as a household telephone set. Since a user can talk without holding the handset in his hand, he can be freed from the fatigue of the hand that occurs after talking for a long period of time or freed from the pain at in the ear due to pressing of the handset against the ear. Since the user is free to use his hands, he can advantageously do something else, while talking to the listener through the telephone. Especially, when this type of telephone set is used in a mobile telephone system located in a car, it provides a great advantage in safety during driving.
A telephone circuit is constituted of a two-wire and a four-wire circuit, and a hybrid circuit is used to carry out the conversion between the two-wire and four-wire circuit.
In this hybrid circuit, however, it is difficult to match the impedance completely, so that the inevitable result is a reflection. Generally, when a conversation is carried out by using a microphone and a loudspeaker which has been placed in a telephone set, a signal loop is formed by an acoustic coupling between the microphone and the loudspeaker and the reflection of the signal of the two-wire/four-wire converting circuit. When a gain of this loop exceeds 1 (OdB), a howling phenomenon is generated, in which conversation is virtually impossible.
Under this condition neither, the level of the speaker can be raised nor the sensitivity of the microphone can be enhanced. Therefore, this telephone set cannot be practically employed. It is an object of the invention, to prevent an acoustic howling by the use of an echo suppresser or an echo canceller, or the like.
The reflection of the signal in the two-wire/four-wire converting circuit results in a considerable time delay in a long-distance telephone, so that it normally causes a so-called echo even though it does not produce the howling, which results in an obstruction for conversation.
The echo suppresser which has been used as a countermeasure to prevent the howling employs a so-called voice-operated system. This system compares the magnitude of speech level between two parties, increases the insertion loss of the smaller one to disconnect the signal path of said smaller one, so the gain of the aforesaid loop does not exceed 1.
This system, however, has a disadvantage that a time lag is inevitably generated in the comparison of the speech levels. As a result of this, the control of the amount of the loss is delayed, and the head of a word may be possibly be split. This system has another disadvantage that a blocking phenomenon may be produced, in which the loss amount cannot be controlled when the system is used at a noisy place and noises continuously enter.
On the other hand, an echo canceller using a digital signal processing system has been recently brought to the public's attention as a result of the progress in techniques which employs low-price semiconductors and easy and sure digital signal processing operation. This echo canceller system of the present invention is free from the disadvantages involved in the echo suppresser as mentioned above. In fact, it is contemplated, as a new application, to be used in the field of a telephone conference system. Known examples of the echo canceller include Japanese patent application laid-open KOKAI No. 62-123837.
The principle of the echo canceller system will now be described with reference to FIG. 2.
In this system, only a signal y(t) is outputted from a loudspeaker 2 and reflected from a wall or the like of a room is cancelled among signals s(t)+y(t) inputted to a microphone 5. Accordingly, the aforementioned loop of the signal is not formed, and the howling is prevented. It is not required, in this system, to consider an insertion loss as different from the voice switch. This allows a simultaneous bidirectional conversation and, the head and the end of the word are not split, ensuring good speech quality.
This type of echo canceller is called an acoustic echo canceller because an acoustic coupling path is assured between the microphone and the loudspeaker. Since this echo canceller processes digital signals, an input signal, is converted into digital form by an A/D converter, and an output signal is reproduced to an analog signal by a D/A converter.
First, an explanation is given for the case in which all the input signals to the microphone 5 are echo sounds from the loudspeaker 2. Namely, it is the case when s(t)=0.
A received signal x(t) from a speech device (not shown) is inputted to the received signal input terminal 1 of an echo canceller unit 3, and fed to an X register 8 through an A/D converter 4. The signal is additionally received from a loudspeaker output signal terminal 15 and transmitted to the loudspeaker 2 via an amplifier 17. A sound emitted from the loudspeaker 2 is transmitted to the microphone 5 as a signal of y(t) due to the reflection from the wall of the room or the like and inputted to a microphone input terminal 16 via an amplifier 18. A path through which the signal x(t) becomes the signal y(t) is called an echo path. Further, the microphone input signal y(t) is converted into a digital signal by an A/D converter 6 and inputted to an adder 11.
The received signal x(t), which is converted to digital form by the A/D converter 4, is sequentially stored in n pieces of X registers 8. In the registers 8, every time one sample of the received signal is stored, each of the received signals is sequentially moved to a respective forward adjacent position, and data stored in the last position is abandoned. In such a way, the data of the received signals of n samples (x(t) to x(t-n+1)) is constantly stored in the n pieces of memories.
Tap coefficient registers 9 are provided to be as many as the X registers 8. A tap coefficient (ho(t) to h n-1(t), n: the number of taps), which is an approximation to the impulse response of the assumed echo path, is stored in the tap coefficient register 9.
A convolution computing element 10 carries out a convolution operation based on the data of the tap coefficient register 9 and the X register 8 inputted thereto. The operation can be expressed by: ##EQU1## A pseudo echo signal y (t) is obtained by subtraction of a subtracter 11 from the input signal y(t) to the microphone 5 and outputted as a subtracting operation result. Thus, the echo signal is cancelled. This result is an estimated error of the echo path and herein called a residual signal e(t) and expressed as follows: EQU e(t)=y(t)-y(t) (2)
This residual signal e(t) is inputted to a correction amount calculator 12, converted into an analog signal by a D/A converter 7 and fed to a communication system as a transmitting signal through a transmitting signal output terminal 14.
The echo canceller modifies the aforesaid tap coefficient, according to an algorithm for reducing the residual signal e(t) 0. Examples of this algorithm which is employable, are well known algorithms such as an LMS method (Least Mean Square Method) or a learning identification method. According to these methods, the tap coefficient is corrected or renewed momentarily based on e(t) and x(t), and the obtained result is applied as a new tap coeffiecient. This will be expressed by the following equations, assuming that the amount of correction of the tap coefficient is assumed to be .DELTA.hi(t). EQU hi(t+1)=hi(t)-.DELTA.hi(t) (3) EQU .DELTA.hi(t)=G.xi(t).e(t) (4)
Where G indicates a correction coefficient, which is a constant in the LMS method, and a value obtained by the following equation, in the learning identification method. ##EQU2##
These operations are performed by the use of the correction amount calculator 12 and an adder 13. In the correction amount calculator 12, .DELTA.hi(t) is successively calculated. The calculated result and corresponding data read from the tap coefficient registers 9 are added by the adder 13. The added result is stored again in an appropriate position of the tap coefficient register 9.
The tap coefficient is estimated, based upon an assumption that only the echo signal y(t) is inputted to the microphone 5. When a sound s(t) (transmitting signal) other than the echo signal y(t) is inputted, that has, in case of y(t)+s(t), the tap coefficient is not estimated correctly, because the tap coefficient as discussed above, is based on only the inputted echo signal (xt). When there is the additional transmitting signal s(t), it is, therefore, necessary to prohibit the tap coefficient from being updated.
For example, the signal levels of the input signal s(t)+y(t) inputted to the microphone 5 and an input signal to the loudspeaker 2, namely, the received signal x(t), are compared. When the power of s(t)+y(t) is larger than the power of y(t) by a predetermined amount, it is judged that there is a transmitting signal s(t) in addition to y(t), and the output data of the correction amount calculator 12 is brought to 0, to suspend temporarily an operation for updating the tap coeffiecient.
Thus, even when there is the transmitting signal s(t), the echo canceller can be stably operated.
The prior art as set forth above, however, involves some problems, which will be discussed hereinafter.
It is difficult to correctly detect the interval for prohibiting the updating operation, due to the properties of a sound. Occasionally, the amount of attenuation by the echo canceller is reduced due to possible delay of the detection or an erroneous detection of the interval. When a user speaks while he is listening to a radio or the like, all sounds other (e.g. sounds from the radio) than the sound (received signal) from the loudspeaker of the echo canceller are disturbances to the echo canceller. This causes an unstable operation.
Since the sound except the sounds from the loudspeaker of the echo canceller can be completely incorporated into the other part of the speech signal, this brings trouble into the speech signal. Especially, it has been a great problem that the sounds inconveniently heared by the other party cannot be reproduced in such a manner that they cannot be eliminated and are heard by the other party of the speech.